Effect of Particle Morphology on Cloud Condensation Nuclei Activity

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Letter

Effect of Particle Morphology on Cloud Condensation Nuclei Activity Muhammad Bilal Altaf, Dabrina D. Dutcher, Timothy M. Raymond, and Miriam Arak Freedman ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00146 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Earth and Space Chemistry

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Effect of Particle Morphology on Cloud Condensation Nuclei Activity Muhammad Bilal Altaf,1 Dabrina D. Dutcher,2,3 Timothy M. Raymond,3 and Miriam Arak Freedman1,* 1) Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States 2) Department of Chemistry, Bucknell University, Lewisburg, PA 17837 3) Department of Chemical Engineering, Bucknell University, Lewisburg, PA 17837 Revised Submission for ACS Earth and Space Chemistry April 30, 2018 To whom all correspondence should be addressed: [email protected]; (814) 867-4267

Abstract

Cloud condensation nuclei (CCN) activation is sensitive to the size, composition, and morphology

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of aerosol particles < 200 nm. By controlling the particle morphology of internally mixed samples

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(i.e. homogeneous vs. phase separated), we have probed the effect of morphology on CCN activity

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using model organic aerosol systems, where ammonium sulfate was mixed with either pimelic acid

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or succinic acid in a 50/50 mixture by weight. Surprisingly, for systems of the same composition,

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but distinct morphology, we observe a noticeable impact on CCN activity. Specifically, a phase

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separated morphology results in activation diameters close to that of ammonium sulfate, while a

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homogeneous morphology yields an activation diameter in between the pure inorganic and organic

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components. Our results suggest that morphology-resolved CCN data may be an important

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parameter to consider in cloud microphysics models to improve predictions of CCN activity of

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complex organic aerosols. For laboratory CCN studies, it is important to control or account for

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atomized solution drying rates which have been shown to affect morphology and ultimately CCN

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activity.

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Keywords: liquid-liquid phase separation, organic aerosol, droplet activation, cloud microphysics,

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climate

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Introduction

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Organic aerosol particles are chemically complex and ubiquitous in the Earth’s

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atmosphere.

Recently, a central focus of the atmospheric science community has been to

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understand the climate forcing effects of these complex systems. For example, the impact of the

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aerosol indirect effect (i.e. the ability of aerosol particles to serve as cloud condensation nuclei,

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CCN) on climate forcing remains highly uncertain.1,2 It is essential to obtain a clear understanding

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of the population of aerosol that serves as cloud condensation nuclei, as CCN play a critical role

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in cloud formation, lifetime, reflectivity, and precipitation.1-4

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The tendency of an aerosol particle toward activation and growth via condensation to

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ultimately form a cloud droplet is controlled by a number of chemical and physical properties of

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the particle. Aerosol particle size and composition are key parameters in CCN activation that have

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received increasing attention in the literature.1-4 The abilities of species such as sulfates and

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abundant inorganic salts to serve as CCN are relatively well understood.1,2

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understanding of the ability of organics to serve as CCN is quite limited and has resulted in a large

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uncertainty in the global modeling of cloud droplet nucleation and indirect forcing.1 Recently,

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within the atmospheric chemistry community, interest has emerged in investigating the role of

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particle mixing state and surface active organics on CCN activity.5-7

However, our

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To better constrain the component of the indirect effect that is related to CCN, global

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models must represent the complexity of organic species and the degree of mixing of individual

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species with a given mass or size.8 Thus, studies of parameters such as particle morphology which

2


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involve understanding the physics and chemistry of aerosol particles are critical. Recently, a

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number of studies have addressed the uncertainty that remains in the factors that contribute to

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hygroscopic growth and CCN activation in mixed particles.5,9 The impact of the presence of

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liquid-liquid phase separation (LLPS) and the role of surface tension on the mechanism of cloud

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droplet formation and observed hygroscopicity parameters have received great interest due to their

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importance for our ability to predict cloud properties.5, 10-12 As shown in our previous work for

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systems that undergo LLPS, we have observed a size-dependent morphology of organic aerosol,

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where small particles are homogeneous and large particles are phase separated.13 We have also

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shown that for some of the systems we have explored, particle morphology can be controlled by

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varying the drying rate particles experience in a given experiment.14 Furthermore, we have

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previously demonstrated that for a given system, phase separated and homogeneous particles are

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mainly spherical in shape and have the same composition.15

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In this work, we apply our results from earlier experiments that allow us to control particle

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morphology for model organic aerosol systems.14 With the capabilities of the CCNC in mind, we

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designed a study to probe the effect of particle morphology on CCN activity. Specifically, we

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have worked with ammonium sulfate mixed with either pimelic acid or succinic acid, and have

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investigated whether the morphology of a given system (i.e. phase separated vs. homogeneous)

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impacts its CCN activity.

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Experimental Methods

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Solution preparation and aerosol particle generation

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Aerosol particles composed of ammonium sulfate (> 99.0%, EMD), pimelic acid (>98%,

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Acros Organics) or succinic acid (>99%, Sigma Aldrich) were generated from aqueous solution.

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Solutions for mixtures of each organic compound mixed with ammonium sulfate (at a 50/50 mass

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fraction) were also prepared. The concentrations of the aqueous solutions (prepared using high-

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performance liquid chromatography grade ultrapure water) ranged from 0.05 wt.% to 0.3 wt.%

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solute. Aerosol particles were generated at a high relative humidity (RH ≥ 96%) and at a

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temperature of 295 K from the aqueous solutions using a constant output atomizer (TSI 3076,

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Shoreview, MN). Nitrogen flow through the atomizer was approximately 1.5 L/min.

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Controlling Particle Morphology

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As discussed in our previous work, we are able to control particle morphology for the

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systems described above by altering the drying rates particles experience in a given experiment.14

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To obtain a homogeneous morphology in the ~50 – 100 nm size regime, we use a diffusion dryer

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filled with molecular sieves (13X mesh size, Sigma Aldrich; drying rate of 87.2% RH/s). Note

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that details of the calculation of drying rates are included in our previous work.14 As we have

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shown previously, all of the particles in the size regime of interest (50 – 100 nm) are homogeneous

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when this drying rate is used.14 A Tedlar® bag (New Star Environmental, Roswell, GA) can be

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used to slow down the drying rate by several orders of magnitude to 0.08% RH/s. This slower

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drying rate yields phase separated (partially engulfed morphology) particles in the ~50 – 100 nm

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size regime. Note that all of the particles in this size regime have a partially engulfed morphology

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when this drying rate is used.14, 16 Thus, by employing this method, we can effectively control the

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morphology of particles used in our CCN studies.

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Measuring Cloud Condensation Nuclei Concentrations

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As seen in Figure 1, once the aerosol particles are generated and undergo drying, we can

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size select the aerosol stream using a differential mobility analyzer (DMA). The distribution of

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particle sizes exiting the DMA is determined by the theoretical transfer function when the particles

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are spherical.17, 18 Based on prior studies, we expect that ammonium sulfate, succinic acid, pimelic

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acid, and homogeneous particles composed of ammonium sulfate mixed with either organic acid

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are nearly spherical.16 The phase separated particles will have a more varied, but still nearly

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spherical shape, with some deviation towards an elliptical shape.16 We have shown, using a variety

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of mineral dust aerosol systems, that the greater the deviation from a spherical shape, the greater

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the polydispersity of diameters transmitted through the DMA compared to the theoretical transfer

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function.19 - 21 As a result, we expect that the transfer function for the phase separated particles

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will be slightly more polydisperse than predicted by the theoretical transfer function. After size

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selection, the aerosol flow is split between a CCNC and a condensation particle counter (CPC).

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The detailed design and operation of the Droplet Measurement Technologies-CCNC (DMT-

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CCNC; Longmont, Colorado) has been previously published.22 Note that the particles are dry

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when they enter the CCNC. As shown in Figure 1, a CPC is included in the experimental setup to

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determine the concentration of particles. With this information, the fraction of particles that

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activated as CCN can be determined as a function of particle size.

Atomizer

Diffusion Dryer

Differential Mobility Analyzer

Sample

Cloud Condensation Nuclei Counter

DMT

Nitrogen

Condensation Particle Counter

Tedlar Bag

116 117 118

CCNC

Figure 1: Schematic of the cloud condensation nuclei counter experiments. The aerosol flow either goes through the diffusion dryer or the bag, depending on the desired drying rate.

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Before investigating the cloud condensation nuclei activity of organic aerosol mixtures, the

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CCNC instrument was calibrated with ammonium sulfate, a system that has been well

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characterized in the literature.3 In each calibration experiment, four different temperature gradient

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values (ΔT) were used to check the linearity between critical supersaturation and temperature

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gradient. A large deviation from linearity indicates possible malfunction in the instrument, such

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as a faulty humidifier. Our experiments resulted in a linear calibration curve between the

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supersaturation and temperature gradient and thus did not indicate any issues with the operation of

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the instrument.

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Results and Discussion

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For the ammonium sulfate CCN experiments, 30 different dry diameter (Dp) values were

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size selected by the DMA in the range of 15-220 nm. We note that we employed this method

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rather than scanning a range of particle diameters to allow for increased averaging time and

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adjustment for particle concentration as the diameter is changed. At each Dp, the total number

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concentration of aerosol particles, CCN, was measured with a condensation particle counter. The

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number concentration of CCN, CCCN, was measured with the CCNC. By combining these

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measurements, the CCN efficiency (activated particle fraction; CCCN / CCN) was calculated from

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the averaged concentrations and a CCN efficiency spectrum of the fraction activated vs. Dp was

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plotted. The CCN efficiency spectrum for ammonium sulfate at a 0.35% supersaturation is shown

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in Figure 2. Multiply charged particles can cause activation spectra to show a plateau before

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exponential growth phase of the curve. Rather than using any of the available charge correction

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algorithms, the data from these extraneous plateaus were eliminated from the data used for the

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sigmoid fits. The points not included in the regression data are shown using an alternate data 6


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marker, “X”, in the spectra of the pure compounds. Using uncorrected data could result in slight

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shift of the curve toward smaller particles (a left-ward shift). It should also be noted that data

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significantly larger than the activation diameter were eliminated from the fit data as is common

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procedure.23, 24 When the standard error of estimate from the regressions is used to calculate a

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95% confidence interval for the activation diameter (Dp50) of the ammonium sulfate, it contains

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the literature value at this supersaturation, which validates the regressions.

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Sigmoid fits were calculated using SigmaPlot’s (Systat Software, Inc.) Dynamic Curve

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fitting function with a five parameter model. Equation 1 shows the form of the calculated

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regression: 𝑓 = 𝑦° +

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𝑎 (1+exp(−

𝑥−𝑥° 𝑐 )) 𝑏

(1)

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where a, b, c, x0 and y0 are regression variables and f is the fraction of particles activated (CCCN/CCN)

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and x is the dry particle size, Dp (nm). Coefficients of determination for all reported fits were

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0.97 or greater.

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The collected data are corrected for doubly charged particles and fit with the sigmoid

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function described in Equation 1. Figure 2 illustrates this process for ammonium sulfate, where

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the closed symbols represent the raw CCN data, the ‘X’ symbols represent the data not included

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in the fit, and the solid lines depict the sigmoid fit to the experimental data which is used to derive

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the activation diameter, Dp50, where 50% of particles activate as CCN. We note that Figure 2

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shows the CCN spectra and sigmoid fit for each individual trial. The Dp50 is derived for each trial

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and the average Dp50 is reported within the figure. We have obtained a Dp50 = 51 (nm) ± 6% for

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ammonium sulfate, the 95% confidence interval for which includes the literature value of 58 nm

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at a 0.35% supersaturation.25, 26 Note that when multiple supersaturations are used, it is useful to

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calculate a  factor to compare the hygroscopicities of different systems.27 Because we have used

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only one supersaturation, we report only Dp50.

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Figure 2: CCN efficiency spectra for ammonium sulfate. The circles represent raw data used for fitting the sigmoidal curve while the ‘X’ represent data points from doubly-charged particles which were removed before fitting the curve but shown for completeness. The three colors represent different trials of the same system.

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Following the ammonium sulfate experiments, the CCN activity of the pure organic

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compounds, pimelic acid and succinic acid, was investigated at a supersaturation of 0.35%.

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Pimelic acid has an activation diameter of 70 nm (σest = 7%), while Dp50 = 58 nm (σest = 4%) for

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succinic acid. Figure 3 summarizes the CCN data for the pure organic compounds.

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179 180 181 182

Figure 3: CCN efficiency spectra for pimelic acid and succinic acid. The circles represent raw data used for fitting the sigmoidal curve while the ‘X’ represent data points from doubly-charged particles which were removed before fitting the curve but shown for completeness. The three colors represent different trials of the same system.

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Since the goal of this work was to probe the effect of particle morphology on CCN activity,

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particles were generated from 50/50 mixtures by weight of each organic compound and ammonium

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sulfate and measured in the CCNC. By controlling the particle drying rate for a given system,

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either a homogeneous or phase separated morphology was obtained and investigated with the

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CCNC. We note that as shown in our earlier work, 50/50 mixtures of ammonium sulfate mixed

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with either pimelic acid or succinic acid yield a partially engulfed morphology upon phase

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separation.13 A phase separated morphology for the 50/50 ammonium sulfate/pimelic acid system

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yielded an activation diameter of Dp50 = 52 nm (σest = 7%), however, for the same system, a

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homogeneous morphology resulted in a Dp50 = 58 nm (σest = 7%). Figure 4 summarizes the CCN

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spectra for ammonium sulfate, pimelic acid, and the 50/50 ammonium sulfate/pimelic acid system

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dried using a Tedlar® bag (TB; phase separated morphology) and diffusion dryer (DD;

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homogeneous morphology). We note that the summary spectra shown in Figure 4 are an average

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of multiple runs for each respective system. The individual dataset for each trial is provided in the

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supporting information (Figure S1). For Figs. 2, 3, S1, and S2, individual data sets are fit to find

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individual Dp50s which are then averaged; for Figs. 4 and 5, individual data sets are combined and

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then fit to find Dp50. As a result of the difference in when the averaging was performed, values

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for Dp50 can differ by 1-2 nm. Note that while a slight difference is observed between the

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activation diameters of ammonium sulfate and the partially engulfed (TB) system, they are within

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experimental error. Based on the results of Rose et al., we expect that correcting for deviations

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from the theoretical transfer function will result in an equivalent activation diameter and an even

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steeper slope of the activation curve for phase separated particles.28

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205 206 207 208

Figure 4: Summary CCN spectra for ammonium sulfate, pimelic acid, and the 50/50 ammonium sulfate/pimelic acid system dried using a Tedlar bag (TB; phase separated morphology) and diffusion dryer (DD; homogeneous morphology).

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For the 50/50 ammonium sulfate/succinic acid system, an activation diameter of Dp50 = 52

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nm (σest = 5%) was obtained for phase separated particles, while a homogeneous morphology

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resulted in a Dp50 = 55 nm (σest = 5%). Figure 5 summarizes the CCN spectra for ammonium

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sulfate, succinic acid, and the 50/50 ammonium sulfate/succinic acid system dried using a Tedlar

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bag (TB; phase separated morphology) and diffusion dryer (DD; homogeneous morphology). Note

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again that while a slight difference is observed between the activation diameters of ammonium

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sulfate and the partially engulfed (TB) system, they are within experimental error. Based on the

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results of Rose et al., we expect that correcting for deviations from the theoretical transfer function

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will result in an equivalent activation diameter and an even steeper slope of the activation curve

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for phase separated particles.28

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221 222 223 224

Figure 5: Summary CCN spectra for ammonium sulfate, succinic acid, and the 50/50 ammonium sulfate/succinic acid system dried using a Tedlar bag (TB; phase separated morphology) and diffusion dryer (DD; homogeneous morphology).

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Surprisingly, the CCNC data for the 50/50 mixtures show that, given two systems of

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identical composition, particle morphology has a noticeable effect on the activation diameter.

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Thus, the efficiency of particles to serve as CCN changes between phase separated and

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homogeneous morphologies. Specifically, the activation diameter of phase separated particles is

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similar to that of ammonium sulfate for the systems studied in this work. As shown in our previous

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work, both pimelic acid and succinic acid mixtures with ammonium sulfate yield a partially

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engulfed morphology upon phase separation and drying.13 Homogeneous particles, on the other

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hand, have an activation diameter that is in between the pure inorganic and organic components.

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Based on the Zdanovskii-Stokes-Robinson (ZSR) Model, we would expect that the κ factors that

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are obtained for the homogeneous 50/50 mixtures would be equivalent to the volume weighted

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average of the components, as shown by

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𝜅ℎ𝑜𝑚𝑜𝑔𝑒𝑛𝑒𝑜𝑢𝑠 =

𝑉𝑜𝑟𝑔 𝑉𝑡𝑜𝑡

𝜅𝑜𝑟𝑔 +

𝑉𝑖𝑛𝑜𝑟𝑔 𝑉𝑡𝑜𝑡

𝜅𝑖𝑛𝑜𝑟𝑔

(2)

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where Vorg, Vinorg, and Vtot are the volumes of the organic component, inorganic component, and

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total, respectively; and org, inorg, and homogeneous are the  values for the organic component,

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inorganic component, and homogeneous particle, respectively.29 Because κ is related to

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diameter, the expected Dp50 for mixtures would similarly fall between that of the two component

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Dp50s.

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In contrast to the results for homogeneous particles, our results for phase separated

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(partially engulfed) particles are surprising and additional studies will need to be performed to

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fully understand the origin of the results. To begin to explain the results for partially engulfed

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particles, we can consider the implications of this morphology on the Köhler curve, given by

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𝑠𝑘 = 𝑛

2𝜎

𝐿 𝑅𝑇𝑟𝑑

3𝑖𝑁𝑠

− 4𝜋𝑛

3 𝐿 𝑟𝑑

(3)

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where sk is the supersaturation,  is the surface tension, nL is the number of moles of water in a

249

liter of solution, R is the ideal gas constant, T is the temperature, i is the van’t Hoff factor which

250

accounts for the dissociation of salts into ions in solution, Ns is the total moles of solute, and rd is

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the radius of the particle.29 In a partially engulfed particle, water vapor has direct access to the

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ammonium sulfate component. With this morphology, it is likely that the water solvates the

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ammonium sulfate prior to the less hygroscopic organic component. For the ammonium

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sulfate/pimelic acid system, it is a reasonable approximation that the pimelic acid is effectively

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255

insoluble when the ammonium sulfate is at the point of activation based on the fact that the

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activation curves for ammonium sulfate and pimelic acid in Figure 4 have little overlap. At the

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point of at which the particle activates, the pimelic acid will therefore not influence the surface

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tension. As a result, the first term in Eqn (3) should remain the same as for an ammonium

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sulfate particle. The amount of soluble material has decreased (considering only the ammonium

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sulfate component), reducing the second term in Eqn (3). As a result, the supersaturation will

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increase, leading to an increase in the activation diameter. However, the pimelic acid also serves

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as a heterogeneous catalyst, which will decrease the activation barrier for activation, lowering

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the activation diameter.30 For ammonium sulfate/succinic acid, a fraction of the succinic acid

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particles are activated at the point at which 50% of the ammonium sulfate particles activate as

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shown in Figure 5. For particles in which succinic acid is not activated, the case is the same for

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pimelic acid. Activation of some succinic acid particles may result in a decrease in surface

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tension. Surface partitioning of organic compounds impacts CCN activation and hygroscopic

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growth of organic aerosol.5, 31-35 Surface-active organics can affect cloud droplet formation by

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inhibiting water uptake, reducing surface tension, and causing activation to occur at a larger

270

diameter. 5, 31-35 These effects result in a reduced supersaturation and reduced activation

271

diameter. From the above discussion, we can conclude that a partially engulfed particle likely

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activates at a smaller diameter than a homogeneous particle, but it is striking that the

273

experimental result shows activation at approximately the same diameter as ammonium sulfate.

274

Additional work needs to be performed to determine why this is the case. We also note that the

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presence of other morphologies (e.g. core-shell) may also impact CCN activation and will be

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explored in future studies.

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An effect of LLPS on CCN activation has been previously reported by Ovadnevaite et al.12

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Bertram and coworkers determined that secondary organic material (SOM) can undergo LLPS

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without the presence of salt as the RH is increased > 90 % because of the difference in solubility

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of the components of the SOM.10, 11 At high RH, the less soluble components of SOM form a new

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phase at the surface of the particle, which lowers the surface tension of the aqueous particle.

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Ovadnevaite et al. hypothesize that the resultant reduction in surface tension leads to much greater

283

numbers of CCN observed in the field than expected.12 In contrast, the aerosol used in this study

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are combinations of an organic compound and a salt, which undergo LLPS as the RH is decreased

285

below a given value due to salting out of the organic compound.

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Conclusions and Atmospheric Implications

287 288

We have investigated the effect of particle morphology on CCN activity. Using our results

289

from earlier work where a size-dependent morphology was observed for aerosol particles

290

composed of organic compounds and salts, we have specifically studied the difference in activation

291

caused by a homogeneous vs. a phase separated morphology. Interestingly, for systems of

292

identical composition, distinct morphologies lead to noticeable changes in CCN behavior. For

293

50/50 mixtures of ammonium sulfate/pimelic acid and ammonium sulfate/succinic acid, a phase

294

separated morphology results in activation diameters close to that of ammonium sulfate, while a

295

homogeneous morphology yields an activation diameter in between the pure components. The

296

data presented in this study indicate that morphology is an important parameter in determining

297

CCN activity for the systems investigated in this work. By accurately assessing and accounting

298

for the effect of morphology, cloud microphysics models will be able to better predict the CCN

299

activity of mixed atmospheric particles containing complex organics. Furthermore, by combining

300

particle composition information and morphology-resolved CCN data, we can compute

15



301

hygroscopicity parameters for multicomponent aerosol particles containing varying amounts of

302

inorganic and organic compounds. To better predict the impact of aerosol particles on cloud

303

properties and the radiative balance of the Earth, our experiments suggest that it is important to

304

investigate whether models should incorporate the complex physical and chemical parameters of

305

aerosol particles that become cloud condensation nuclei.

306 307

Acknowledgements

308

We are grateful for support from the CAREER program of NSF (CHE-1351383) for the collection

309

of data and NSF grants AGS-1723290 (M. B. A., M. A. F.) and AGS-1723874 (D. D. D., T. M.

310

R.) for the analysis and interpretation.

311 312

Supporting Information

313

includes individual activation curves and fits for the internally mixed systems of ammonium

314

sulfate/pimelic acid and ammonium sulfate/succinic acid

315 316

References

317 318 319 320 321 322 323 324 325 326

1. Stocker, T. F.; Qin, D.; Plattner, G.-K.; Tignor, M. M. B.; Allen, S. K.; Boschung, J.; Nauels, A.; Xia, Y.; Bex, V.; Midgley, P. M. Climate change 2013: the physical science basis.: Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change; Cambridge University Press: Cambridge, 2013. 2. Wallace, J. M.; Hobbs, P. V. Atmospheric science: an introductory survey; Academic Press: London, 2006.

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34. Blower, P. G.; Ota, S. T.; Valley, N. A.; Wood, S. R.; Richmond, G. L. Sink or Surf: Atmospheric Implications for Succinic Acid at Aqueous Surfaces, The Journal of Physical Chemistry A, 2013, 117(33), 7887–7903. 35. McNeill, V. F.; Ariya, P. A. (Eds.) Atmospheric and Aerosol Chemistry, Springer: Berlin, Germany, 2014.

455 456 457

For TOC only: 1:1 inorganic/organic phase separated

fraction activated

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

homogeneous

458

dry diameter

459 460

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Page 20 of 20

Effect of Particle Morphology on Cloud Condensation Nuclei Activity

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